JPS63229360A - Gas sensor element - Google Patents

Abstract

PURPOSE:To improve the response of a sensor element and to prevent the generation of a crack in a cap due to heating, by interposing an insulating buffer layer between the cap for controlling the diffusion of gas and a heater. CONSTITUTION:A heater 6 heating a sensor to high temp. is mounted on the gas diffusion control cap 5 of a gas sensor element and an insulating buffer layer 7 such as an alumina layer of a semi-sintered state (high in a whole void ratio) is provided between both of them. A current of 10mA order flows to a heater circuit and leaks to an electrode circuit to exert effect on a sensor response speed but, by interposing the buffer layer 7, the generation of leak current is prevented to make it possible to improve response. Further, by forming the buffer layer 7 from the alumina layer high in the whole void ratio, the difference in coefficient of thermal expansion is absorbed to relax the heat cushion of the cap 5 and the generation of a crack can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a limiting current type gas sensor element that detects the concentration of a specific gas contained in a gas.

(Prior Art) A limiting current type gas sensor using a solid electrolyte is known as a sensor for detecting gas concentration in gas. As an example, FIG. 3 shows a sectional view of an oxygen sensor element. In the figure, 1 is a solid electrolyte, and plate-shaped stabilized zirconia ((ZrOz) (Yz(h), ,
A porous platinum electrode anode 2 and a cathode 3 are formed on both sides of the porous platinum electrode anode 2 and a cathode 3. For example, a partially stabilized zirconia (( ZrOz) (Yz(h
) 0.03) or the like, and a heater 6 constructed by applying and baking a paste of gold, platinum, etc., is attached to the upper surface of the cap. When the sensor is heated to a high temperature by the heater 6 and a voltage is applied between the electrodes, the oxygen contained in the gas is reduced to oxygen ions at the cathode 3, and these oxygen ions fill the oxygen ion vacancies in the solid electrolyte 1. The oxygen ions are transferred to the anode 2 through the oxygen ions, and a current using oxygen ions as carriers flows due to a so-called bombing effect.

The diffusion of oxygen is controlled by the micropores 4 provided in the cap 5, and this current reaches a rate-determining rate and becomes constant in a certain region of the applied voltage, producing a flat portion in the current-voltage characteristics. The current in this flat portion is called a limiting current, and since this limiting current value depends on the oxygen concentration, the oxygen concentration can be known by measuring the limiting current value. In addition, in the case of a hydrogen sensor element as a gas sensor, for example, the transport direction of hydrogen ions is opposite to that of oxygen ions, so the gas diffusion control cap is provided on the anode side, etc. Depending on the type of detection gas, the sensor element may be partially Although there are some differences, since the structure is the same, the oxygen sensor element will be mainly described below.

(Problem to be solved by the invention) An important characteristic of a gas sensor is the time it takes for the output of the gas sensor to change in response to a change in the gas concentration in the gas, that is, the responsiveness, and the gas sensor operates sensitively. For this purpose, further improvement in response speed is desired. As with conventional sensors, partially stabilized zirconia is usually used for the cap, and when a heater pattern is formed directly on the top surface of the cap, the heater has self-temperature control (for example, 400
At °C (ZrOz) (Y2O2) 0.03 is 4.7X
Although it has the advantage of not requiring a heater control means because it has a conductivity of 10-6 (Ω-1cm-1), if you try to raise the sensor temperature to improve response, the oxygen in the zirconia cap The temperature cannot be raised because ionic conductivity occurs and a short circuit occurs between the heater patterns.

Further, since the heater is directly attached to the top surface of the cap as described above, heating of the sensor element causes thermal distortion, which tends to cause cracks in the cap, which may even lead to the heater being cut off.

(Means for Solving the Problems) The present invention was made to solve the above problems, and includes a semi-sintered alumina layer (all pores are serious) between the cap 5 and the heater 6. The sensor element is provided with an insulating buffer layer made of a mullite layer, a cordierite layer, or the like, and further provided with an extended portion of a cathode lead on the side surface of the cap.

(Function) For example, in the oxygen sensor mentioned above, the heater circuit has 10
A current of 10 to 100 orders of magnitude flows through the electrode circuit on the order of mA, but partially stabilized zirconia is normally used for the cap 5 because of its excellent heat resistance.
This is because at high temperatures, the specific resistance is relatively small and it becomes somewhat conductive, so the current from the heater circuit leaks to the electrode circuit, and this leakage current is superimposed on the output current of the electrode circuit, which affects the response speed of the sensor. I'll come. Therefore, as described above, interposing an insulating layer between the camp and the heater eliminates leakage current and improves the response speed. Furthermore, by providing an extended portion on the cathode lead, it is possible to promote the release of the charge charged on the cap, so that the response can be further improved. It is desirable that this insulating layer has the same coefficient of thermal expansion as the cap and is highly heat resistant, but since there is no suitable material, an insulating buffer layer such as a semi-sintered alumina layer with high resistivity is used. Furthermore, by forming an alumina layer with a large total porosity, it becomes a buffer layer that absorbs differences in thermal expansion coefficients and alleviates the thermal shock of the cap, thereby making it possible to prevent cracks from occurring in the cap.

(Example) FIG. 1 is a cross-sectional view of an oxygen sensor element as an example of the gas sensor according to the present invention, in which an alumina layer 7 of a semi-sintered body (all pores are important) is insulated between a cap 5 and a heater 6. It is provided as a buffer layer as well as a buffer layer. The rest of the structure is the same as that of the conventional example shown in FIG. 3, so the same parts are indicated by the same symbols and the explanation will be omitted. The alumina layer 7 contains Si in Af203.
Mix 1 to 5% of O□ and 1 to 5% of CaO, and add 14% of this.
A semi-sintered body with a density of 40 to 70% was obtained by sintering at 00°C for about 3 hours. This has excellent heat resistance and insulation properties, so no leakage current flows, and at the same time, it can act as a buffer layer to alleviate the thermal shock of the camp 5, thereby preventing the cap from cracking due to heating.

FIG. 2(A) is a sectional view of an oxygen sensor element according to another embodiment of the gas sensor element according to the present invention, and FIG. 2(B) is a perspective view of the oxygen sensor element.

In the figure, a lead extension 8a is provided on the side surface of the cap 5 using platinum paste on the lead 8 of the cathode 3, and the other structure is the same as that in FIG. 1, and the same parts are indicated by the same symbols. Regarding the example shown in FIG. 2, when the sensor temperature was heated to 450°C by the heater 6 and the oxygen concentration changed from 0 to 21%, the 90% response speed was 20 seconds without the alumina layer 7. However, by providing the alumina layer 7, the time was reduced to 8 seconds.

The above also applies to gas sensor elements other than oxygen sensors.

(Effects of the Invention) As described above, by providing a gas sensor element according to the present invention, the responsiveness of the sensor element is improved, and at the same time, it is possible to prevent cracks from occurring in the cap due to heating and furthermore, from cutting off the heater.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of an oxygen sensor element according to an embodiment of the gas sensor element according to the present invention, FIG. 2(a) is a sectional view of an oxygen sensor element of another embodiment, and FIG. ) is a perspective view, and FIG. 3 is a sectional view of a conventional example of an oxygen sensor element. 1: Solid electrolyte, 2: Porous electrode (anode), 3: Porous electrode (cathode), 5: Cap body for gas diffusion control), 6: Heater, 7: Insulating buffer layer, 8a: Lead extension part . Agent Patent Attorney Takeuchi 9 Figure 2 (A) (B)

Claims (2)

[Claims] (1) In a limiting current type gas sensor element in which a heater is attached to a gas diffusion control cap that covers one electrode of porous electrodes provided on both sides of a gas ion-conducting solid electrolyte, there is a gap between the heater and the cap. A gas sensor element comprising: an insulating buffer layer interposed between the gas sensor element and the insulating buffer layer. (2) The gas sensor element according to claim 1, wherein an extended portion of the lead of the porous electrode is formed on a side surface of the cap.